2009 Amphibian Surveys - The City of Portland, Oregon

51 Pages·2009·1.14 MB·English

by  Katie Holzer

Amphibians in the City
Presence, Influential Factors, and
Recommendations in Portland, OR

Katie Holzer
City of Portland
Bureau of Parks and Recreation
Bureau of Environmental Services
August 2009
Introduction

Background
We are currently in the midst of the largest extinction of species on Earth in 65 million years
(Myers & Knoll 2001, Baillie et al. 2004). Although this crisis is affecting nearly all taxa,
amphibians are being hit particularly strongly, as one in three amphibian species are threatened with
extinction (Pounds et al. 2006). Amphibians comprise frogs, salamanders, and caecilians, but in the
Pacific Northwest of the United States we have only frogs and salamanders.
There are some unique amphibian characteristics that are likely contributing to their rapid
decline: 1) Amphibians have moist, permeable skin that makes them sensitive to pollution and
prone to drying out (Smith & Moran 1930). 2) Many amphibians require multiple specific habitats
such as ponds for egg laying and forests for the summer dry months. These habitats must be
individually suitable for amphibians as well as connected to each other for populations to be
successful (Bowne & Bowers 2004). 3) Many amphibians exhibit strong site fidelity where they
will attempt to return to the same area again and again, even if the area is degraded and/or new
areas are constructed (Stumpel & Voet 1998). 4) Chytridiomycota is a fungus that is transmitted by
water and is rapidly sweeping across the globe taking a large toll on amphibians (Retallick et al.,
2004). The fungus infects the skin of amphibians and has recently arrived in the Pacific Northwest.
All of these factors are contributing to the sharp decline of amphibian populations around the world.
The current loss of amphibians is sure to have profound effects on many ecosystems.
Amphibians play vital roles in food chains and nutrient cycles. In the Willamette Valley,
amphibians are a major consumer of aquatic and terrestrial invertebrates, and they are an important
food source for some snakes, fish and birds. Amphibians also play a unique role in many
ecosystems because they act as sinks for nutrients such as calcium (Burton & Likens 1975). Also,
amphibians' sensitivity to environmental change allows them to act as "canaries in the mine shaft",
that is, they act as indicators of factors that may affect other taxa in the future. If amphibians are
disappearing from our watersheds, it is likely that ecosystem health is declining and that other taxa
will follow.
Historically much of Portland was wetland, but as it developed, most of the wetlands have
been filled in leaving small pockets of habitat that are suitable for pond-breeding amphibians. This
change in habitat has likely affected amphibian populations, but little is known about present
amphibians in Portland. What species live where, and with what density? Why do they live where
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they live? How can we make conserved and restored habitat more suitable for amphibians? These
questions were the focus of this project. Studying these questions will help us to better understand
the amphibians in this area and how we can improve their populations.

Goals
The first goal of this study was to determine the presence and abundance of amphibians in
Portland, OR. This includes determining the locations and relative densities of each species. To do
this, I surveyed natural and developed areas owned by the City of Portland, Metro regional
government, or privately owned where amphibians had been reported, or where it was of interest to
the City to know if amphibians are present.
The second goal was to investigate the factors that contribute to the presence and density of
these species: to discover why they live where they live. By knowing what factors are important for
amphibians in this area, we will be able to better manage our city to benefit amphibians. To isolate
influential factors, I surveyed each area for several factors (the number of factors differs depending
on the life-history of the amphibians present at the site), and analyzed which factors were influential
to the presence and density of amphibian species.
The final goal was to give recommendations for restoration projects so that conserved and
restored areas are beneficial for amphibians. By identifying where species are present and what
factors are important for them, we will be able to make future area planning and restoration
decisions that are more likely to benefit our amphibians.
Each of these goals is presented as a research question in the following sections. Each of
these sections is divided further in to sections by amphibian life-history: terrestrially-breeding and
pond-breeding. As a note, I also found one species of stream breeding amphibian in Portland: the
pacific giant salamander—Dicamptodon tenebrosis. However, I only found it in low numbers at one
site (Forest Park), and I was not able to compare factors across sites to determine what factors are
important for the species. The remainder of my report will focus only on the terrestrially- and pond-
breeding amphibians in Portland.

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1. What amphibian species are present where in the
City, and in what densities?

Terrestrially-breeding amphibians

Methods
I surveyed 18 sites from four watersheds between September 2008 and May 2009. The
watersheds and the number of sites that I surveyed in each are as follows: the Columbia Slough
Watershed (six sites), the Johnson Creek Watershed (six sites), the Tryon Creek Watershed (three
sites), and the Willamette Watershed (three sites). At each site I conducted a time-constrained
search for terrestrially-breeding amphibians.
After becoming familiar with each site while conducting transects (described in question 2),
I searched for 30 minutes at each site for terrestrially-breeding amphibians in the areas that were
seemingly best-suited for them. I conducted these searches only after it had been raining for at least
36 hours because this is when the amphibians are most likely to be found above ground (Wyman,
1988).

Results

I found three species of terrestrially-breeding amphibians in Portland (Figure 1): the Oregon
salamander—Ensatina eschscholtzii, the western red-backed salamander—Plethodon vehiculum,
and Dunn‟s salamander—Plethodon dunni. Table 1 and Figure 2 show at which sites I found each
species present. Out of my 18 sites, I found Oregon salamanders at seven sites, red-backed
salamanders at five sites, and Dunn‟s salamanders at two sites. I did not find any terrestrially-
breeding salamanders in the Columbia Slough watershed. In the Johnson Creek watershed I found
Oregon salamanders at most sites, and red-backed and Dunn‟s salamanders only at one site (Powell
Butte). In the Willamette watershed I found Oregon and red-backed salamanders at two sites, in one
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of which I also found Dunn‟s salamanders. In the Tryon Creek watershed I found red-backed
salamanders at two sites, and Oregon salamanders at the other.

Pond-breeding amphibians

Methods

I sampled each of 83 ponds in 21 sites from four watersheds eight times between May 2008
and August 2009. The watersheds and the number of ponds that I sampled in each are as follows:
the Columbia Slough Watershed (34 ponds), the Johnson Creek Watershed (35 ponds), the Tryon
Creek Watershed (7 ponds), and the Willamette Watershed (7 ponds).

Sampling of eggs
My egg mass surveys were time-constrained searches of randomized transects in and across
each pond. I started each search at a randomized location on the north/south axis of the pond; I
determined this location using a random number table where 0 is the far north end and 9 is the far
south. I used a coin toss to determine whether to start at the east side and move west or to start at the
west side and move east. Once I crossed the pond I moved 1m south and continued in the opposite
direction; if I reached the south end of a pond I continued at the north end of the pond. Figure 3
shows my transect method.
I searched each pond for 20 minutes, looking for egg masses on the water surface, on the
pond bottom, and attached to vegetation. For each egg mass I recorded the species, the approximate
number of eggs, the developmental stage of the eggs, and the type of object to which the mass was
attached. If there were fewer than ten eggs in a mass, I counted each egg individually. If there were
between ten and 100 eggs I estimated to the nearest 5, and if there were more than 100 I estimated
to the nearest 50.

Sampling of tadpoles and larvae
I walked the perimeter of each pond 1m in from the edge. Every three steps I dipped an
aquarium net (opening is approximately 600cm2) into the water. The net entered the water at a full
arm‟s length at a 45° angle to my direction of movement (halfway between my front and my side
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facing the center of the pond). I pulled the net straight towards my body at a depth of ~0.5m. I
identified every amphibian that I encountered to the species level, as well as identifying other
organisms that I found in the pond to a coarse level. I recorded how many times I dipped the net in
each pond to obtain a measure of amphibian density.

Results

I found six species of pond-breeding amphibians (three frogs and three salamanders, Figure
4) in Portland: pacific chorus frog—Pseudacris regilla, red-legged frog—Rana aurora, American
bullfrog-Lithobates catesbeianus, long-toed salamander—Ambystoma macrodactylum, northwestern
salamander—Ambystoma gracile, and rough-skinned newt—Taricha granulosa. Table 2 and Figure
5 show where I found each species breeding in the city. Out of my 83 ponds, I found each species
breeding in the following number of ponds (Figure 6 shows these results):
chorus frogs—31 pond long-toed salamanders—31 ponds
red-legged frogs—17 ponds northwestern salamanders—8 ponds
bullfrogs—16 ponds rough-skinned newts—24 ponds

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2. What factors are influential for amphibians in the
City?

Terrestrially-breeding amphibians

Methods

Transects
I conducted ten transects surveying for amount and type of ground cover as well as number
and type of cover objects at each of the 18 sites at which I surveyed for terrestrially-breeding
salamanders in questions 1.
I followed a foot-path through each site and conducted a transect every 100m along the path
until I had conducted 10 transects. Each transect was in a direction perpendicular to the path and
was 50m long and 2m wide (Figure 7). In each transect I recorded the percent of cover at one meter
height using visual estimation as well as the dominant type of ground cover. I also recorded the
number of rocks and logs that I encountered in each transect. During each transect I stopped every
1.5m and brushed aside the ground cover of a 0.1m2 area in search of salamanders. When I found a
salamander I recorded the type of cover under which I found it as well as the number of steps it was
from the foot path.

Analyses
I conducted regression analyses between the number of terrestrial salamanders found at a
site during the 30 minute surveys (described in question 1) and each of the following factors: 1) the
number of transects dominated by—a) grass, b) leaves, c) needles, or d) ivy, and 2) the mean for
each site of—a) the number of logs, b) the number of rocks, c) the number of rocks and logs
combined, and d) the percent of ground cover. Note: when I say “grass” in this section, I am not
referring to all grasses, but rather to “lawn-type” grass that is no more than 25cm tall.
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I also conducted a t-test comparing the mean number of rocks, number of logs, number of
rocks and logs combined, and percent ground cover in sites where terrestrial salamanders were
present vs. where they were absent.

Results

Ground Cover Type
I found a general positive trend between the number of terrestrial salamanders at a site and
the amount of leaves, needles, and ivy at a site—none of these trends were significant. I found a
strong negative trend between the amount of grass at a site and the number of terrestrial
salamanders that I found there (R2=0.431, p=0.008, Figure 8). Therefore, terrestrial salamanders
seem to be in higher densities at sites that have more leaves, needles, and/or ivy on the ground and
less grass.

Amount of Ground Cover
I found a general positive trend between the amount of ground cover and the number of
terrestrial salamanders at a site, although this trend was not significant. I also in general found there
to be greater ground cover at sites where terrestrial salamanders were present than where they were
absent, but this trend was also not significant. Terrestrial salamanders were present at all sites with
ground cover >57%. Therefore, high amounts of ground cover seem to be beneficial for terrestrial
salamanders.

Cover Objects
I found a general positive trend between the number of cover objects and the number of
terrestrial salamanders at a site, although this trend was not significant (example: Figure 9). I also in
general found more rocks and logs at sites where terrestrial salamanders were present than at those
where they were absent, but this was also not significant. Therefore, cover objects seem to be
beneficial for terrestrial salamanders, but are definitely not the only factors influencing their
distribution. Figure 10 shows under which cover objects I found each salamander. The most
frequent cover object for Oregon salamanders was logs, for red-backed salamanders it was cement,
and for Dunn‟s salamanders it was rocks. There was a significant difference in the number of type
of cover object preferred by Oregon salamanders and red-backed salamanders where Oregon
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salamanders chose logs more often than red-backed salamanders, and red-backed salamanders
chose cement more often than Oregon salamanders.

Distance from Path
I found salamanders at both the minimum and maximum distances that from foot paths that I
searched (1.5m and 50m, respectively). There does not seem to be any pattern in where salamanders
are present in relation to the paths (Figure 11).

Pond-breeding amphibians

Methods
Sampling of Factors
The list of sampling factors was constructed by a group of ecologists as factors that were
likely to affect pond-breeding amphibians. I sampled the following factors for each pond three times
during the summer of 2008, and five times during the spring and summer of 2009: pH, nitrates,
nitrites, dissolved oxygen, bottom temperature, surface temperature, depth, area of pond, clarity,
percent aquatic vegetation, percent refugia, percent shading from above, percent shading from
surface, surrounding vegetative cover, surrounding cover objects, distance to forest, distance to
running water, distance to another pond, seasonal or permanent, age, and man-made or natural.
Following is a brief description of how I measured each of these factors.
Collecting a water sample—I collected an integrated water sample from the deepest point of
the pond, or from a point 1.5m deep if the pond was deeper than 1.5m. To collect an integrated
water sample I inverted an empty 25mL vial and submerged it in the pond to the bottom. Then I
slowly turned it upright as I brought the vial towards the surface to obtain a sample of water that is
representative of all depths.
pH—I dipped a colorpHast® pH-indicator strip in the water sample for two seconds and read
the pH after 2 minutes.
Nitrates and nitrites—I dipped an Industrial Test Systems‟ nitrogen test strip in the water
sample for two seconds and read the results after 60 seconds.
Dissolved oxygen—I snapped a CHEMets® dissolved oxygen vacuole in the water sample
and read the results after 10 minutes.
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Bottom and surface temperature—I held a glass thermometer at the bottom of the deepest
point of the pond (or at a location that was 1.5m deep if the pond was deeper than 1.5m) for 30
seconds to obtain a bottom temperature. I held the thermometer just under the surface of the water
for 30 seconds at the same location for the surface temperature.
Depth—I recorded the depth of each pond at the deepest point for ponds <1.5m deep.
Area—I estimated the area of each pond by pacing the borders. If the pond was larger than
1,000m2 I used GoogleEarth® to estimate the area of the pond.
Clarity—I measured the clarity of each pond on a scale of „1‟-„5‟ (with „1‟ being cloudy and
„5‟ being clear). To do this, I stuck a ruler into the water until I could no longer see the tip. I scored
a „1‟ if this distance was <5cm, a „2‟ if it was between 5 and 10cm, a „3‟ if it was between 10 and
15cm, a „4‟ if it was between 15 and 20cm, and a „5‟ if it was >20cm.
Percent aquatic vegetation—I measured the percent of aquatic vegetation present in the
pond by visually dividing the pond into 25 equal segments, and counting how many of these
segments were dominated by aquatic vegetation.
Percent refugia—I measured the percent refugia in the pond using the same method
described above in percent aquatic vegetation. Refugia include branches, sticks, and plants with
areas for tadpole and larvae to hide.
Percent shading from above—I measured the percent of shading on the pond due to trees,
bushes, shrubs, and grasses using the same method described above in percent aquatic vegetation.
Percent shading from surface—I measured the percent shading on the surface of the pond
due to lily pads, duckweed, and other surface plants using the same method described above in
percent aquatic vegetation.
Surrounding vegetative cover—I surveyed the10m surrounding the pond and classified the
area on a scale of „1‟-„5‟ with a „1‟ indicating that the ground is almost completely exposed at a
height of 1m or below (0-20 percent covered) and a „5‟ indicating that the ground is almost
completely covered at a height of 1m or below (80-100 percent covered).
Surrounding cover objects—I again surveyed the 10m surrounding the pond, and classified
the area on a scale of „1‟-„5‟. I scored a „1‟ if I encountered 0-2 cover objects (rocks and logs), a „1‟
for 3-4 cover objects, a „3‟ for 4-5 cover objects, a „4‟ for 6-7 cover objects, and a „5‟ for 8 or more
cover objects.
Distance to forest, running water, and another pond—For each of these factors, I paced the
distance from the pond to the closest feature of the specified type. I defined a forest as an area with
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